The present invention relates to a liquid crystal composition of the guest-host type for use in a liquid crystal display element which is able to color display.
Liquid crystal compositions of the guest-host type contain pleochroic dyes as a guest which are dissolved in host liquid crystal compositions. The pleochroic dyes are required to have a high order parameter (hereinafter referred to as S) in the host liquid crystals.
The order parameter is defined by the equation (I); the parameter can be obtained by equation (II) experimentally. In the equations, item of cos.sup.2 .theta. is a statistic average, .theta. is an angle between an absorption axis of a dye molecule and an orientation direction of a host liquid crystal, and A// and A.perp. denote absorbances by the dye molecules to the polarized lights, one being directed to in parallel to the orrientation direction of the liquid crystal molecules and the other being directed to parpendicularly to the orientation direction of the liquid crystal molecules, respectively. ##EQU1## S represents a parallelism of the absorption axis of the dye molecule with respect to the orientation direction of the liquid crystal, which governs a display contrast of a display device of the guest-host type. In case of pleochroic dyes having parallel dichroism, as the value of S approaches 1, the degree of color retention of relatively colorless portion becomes small so that a bright display with a high contrast can be possible. The relationship between the molecular structures of the pleochroic dyes and their various properties has not been studied sufficiently. Thus, it is very difficult to select pleochroic dyes having higher S values and a desired hue from a number of known dyes. It is also a difficult job to presuppose the nature of appropriate dyes among the known dyes.
As mentioned above, the S value is the most important parameter for characterizing the pleochroic dyes. In addition to that parameter, a large solubility of the dyes to the host liquid crystal is an important characteristic from the practical point of view. Into the molecules of dyes are often introduced as a so-called auxochromous group the following dialkylamino group, ##STR3## (R's are alkyl groups such as methyl or ethyl group).
When R's in the dialkylamino groups are lower alkyl groups such as methyl group or ethyl group, such pleochroic dyes do not always have a sufficient solubility to the liquid crystal composition. In this case, the solubility can be increased by extending the length of the alkyl group in the dialkyl group to the length longer than butyl group.
This fact has been known in the field of guest-host liquid crystals. However, pleochroic dyes having such the long chains have small S values compared with those having methyl or ethyl groups, though their hues do not change.